Heat the oil in a medium saucepan. Add onion and garlic and sauté for 2 minutes. Add curry powder and ginger and mix well for 2 minutes. Add carrots and yams and stir so they get coated with spices. Add vinegar, salt, apple and stock. Cook on low heat until yams and carrots are tender (about 30-40 min). Add nut butter. Process in blender until creamy (or use hand immersion blender if you like). Add the corn kernels and let simmer for a few minutes